Prob b/c you have multiple tns* files in that directory - we read in more than just tnsnames.ora. Do you have any backup copies of said file?
As a matter of fact, I DID have a tnsnames.ora.bak in the folder with tnsnames.ora, So I deleted it - and you are exactly right, the extra entries disappeared.
One remaining question - WHY do you read other files besides the REAL tnsnames.ora?
Good question - because SQL*Plus does. Someone opened a bug that we weren't following that behavior, so now we do.